Field of the Invention
The present invention relates to an organic light emitting element, a display apparatus, an image information processing apparatus, a lighting apparatus, an image forming apparatus, and an exposing apparatus.
Description of the Related Art
An organic light emitting element (organic electroluminescence element or organic EL element) is an electronic element including a pair of electrodes and an organic compound layer placed between the pair of electrodes. An electron and a hole are injected from the pair of electrodes, and then the electron and the hole recombine in the organic compound layer to produce an exciton of a luminous organic compound. The organic light emitting element emits light upon return of the exciton to its ground state.
In the organic light emitting element, at least one organic compound layer is placed between the electrodes. As one layer constituting the organic compound layer, there is known an electron injection layer that serves to inject or transport an electron injected from a cathode into an emission layer. For example, an alkali metal or alkaline earth metal derivative has been widely known as a constituent material for the electron injection layer. It is because each of the alkali metal and alkaline earth metal derivatives is a material having a small work function and shows a good electron injection property that any such derivative is used as the constituent material for the electron injection layer. However, it has been known that each of the alkali metal and alkaline earth metal derivatives is a material that easily reacts with water. Accordingly, an organic light emitting element containing any one of the alkali metal and alkaline earth metal derivatives as a constituent material for its electron injection layer is affected by moisture in the air. Therefore, at present, the organic light emitting element needs to be stringently sealed so that the organic light emitting element may not be exposed to the moisture in the air. Meanwhile, in order that the effect due to the moisture in the air may be overcome, various researches and developments have been conducted on a method of stably driving the organic light emitting element even in the air except the sealing.
As one method of improving the stability of the organic light emitting element against the moisture in the air, for example, there is known a method involving introducing Compound a-1 having an electron donor property and Compound a-2 having an electron acceptor property shown below into the electron injection layer like U.S. Patent Application Publication No. 2005/0110005.

In the electron injection layer of the organic light emitting element of U.S. Patent Application Publication NO. 2005/0110005, the compound having an electron donor property (D molecule) donates an electron to the compound having an electron acceptor property (A molecule), whereby charge (an electron) is generated. In addition, a strong interaction occurs between the D molecule and the A molecule to produce a polarized DA complex. Thus, the organic light emitting element of U.S. Patent Application Publication NO. 2005/0110005 can perform the injection of an electron.
J. Org. Chem. 2008, 73, 445-450 discloses Viologen Compound b-1 shown below as a compound having a high electron donor property.

However, the HOMO of Compound a-1 (compound having an electron donor property) is deep and hence the energy level of the DA complex to be formed in the organic light emitting element of U.S. Patent Application Publication No. 2005/0110005 is deep. Therefore, when Compounds a-1 and a-2 are used as constituent materials for the electron injection layer constituting the organic light emitting element, an electron can be accepted from an electrode (cathode) but a problem occurs in that a barrier for the injection of an electron into an emission layer is large and hence good light emission is not obtained. Meanwhile, Viologen Compound b-1 disclosed in J. Org. Chem. 2008, 73, 445-450 has a high electron donor property but its availability as a constituent material for an organic light emitting element has not been suggested. In addition, Viologen Compound b-1 itself has a large molecular weight and hence it has been difficult to subject the compound to sublimation purification.